The antennas that equip current secondary surveillance radar (SSR) systems are generally large antennas (4 to 9 m wide) which conventionally possess 3 transmission/reception channels: a channel referred to as “Sum channel” or “Σ channel”, a channel referred to as “Difference channel” or “Diff channel” or else “Δ channel”, and a complementary channel referred to as “Control channel” or “Cont channel” or else “Ω channel”. By virtue of their design, these antennas exhibit a virtually perfect similarity between their transmission characteristics (at 1030 MHz) and reception characteristics (at 1090 MHz). These antennas advantageously allow transmissions to take place over one or other of these channels.
It is common practice for the Σ channel to be the only channel used by the radar to implement the exchanges between the transponders on board aircraft and itself, notably the exchanges in S mode or in IFF mode. The Δ channel is on the other hand used simply in reception to implement the error function allowing the true azimuth of the transponder in question to be precisely determined. With regard to the Ω channel, this allows the signals received by the side lobes of the Σ channel to be identified and, consequently, to be ignored (side-lobe suppression function or “SLS”).
Furthermore, with regard to the operation of these radar systems in S mode or in IFF mode, the standards defined by the International Civil Aviation Organization (“ICAO”), notably in Annex 10, Volume 4, together with those corresponding to the NATO standard STANAG 4193 (1st part) define the signals to be sent out in order to obtain the response from the transponder of an aircraft in the main lobe of the interrogator. It is recalled here that the S mode corresponds to a dedicated mode of interrogation related to civilian aircraft, whereas the IFF mode is a dedicated mode of interrogation relating to military aircraft.
In S mode, the secondary surveillance radar operates in tracking mode, namely where the position of a given aircraft (in azimuth azimuth and distance), for a given antenna rotation, is determined, predicted, by extrapolation of its previous position determined during the detection of this aircraft on the preceding rotation. In IFF mode, the position of a given aircraft (in azimuth and distance), for a given antenna rotation, can be supplied either as in S mode, but also using another sensor such as the primary radar or an optronic sensor for example.
The 3 antenna channels are normally used as follows:
a) with regard to the transmissions (at 1030 MHz):                the Σ channel is dedicated to the interrogation via the main lobe of the antenna, in S mode or in IFF mode, of any transponder (i.e. of any aircraft) localized within the area covered by the main lobe.        the channel “Cont”, or Ω, is employed in order to block the responses from the transponders localized within a region outside of the main lobe and which are however capable of receiving the interrogations transmitted over the Σ channel by the side lobes of this channel. For this purpose, an interrogation signal is transmitted by the channel “Cont” (SLS channel interrogation function or “ISLS”).        
b) with regard to the receptions (at 1090 MHz):                the Σ channel is dedicated to the detection and to the decoding via the main lobe of the antenna, in S mode or in IFF mode, of the responses from any transponder (i.e. from any aircraft) localized within the area covered by the main lobe.        the Ω channel is employed in order to reject the responses of the transponders localized within a region outside of the main lobe and whose responses are received over the Σ channel, by the side lobes of this channel (SLS channel response suppression function or “RSLS”).        the Δ channel is used for the Monopulse processing, in order to precisely determine within the main lobe from which transponder the response originates and also for implementing the “RSLS” function.        
It should be noted that in the application of the OACI and STANAG standards, the ISLS function is normally implemented by aircraft by considering the received radar signals. This results in the transponders necessarily responding if, for the azimuth in question:GSum·PInterrogat ion−GCont·PISLS≧9 dBwhere G is the gain of the antenna at the azimuth of the aircraft and P the power of the interrogations transmitted over Sum (Σ) and Control (Ω). The transponders do not respond if:GSum·PInterrogat ion−GCont·PISLS<0 dB
Between the 2 thresholds (0 dB and 9 dB) and depending on the standards applied, the transponder may or may not respond. Considering the differences in between 0 dB and 9 dB, this is commonly referred to as a “grey area”.
It should also be noted that, in a similar fashion, the RSLS function is implemented by the radar, by considering the signals received from the transponder in question.
The result of this is that the responses received for the azimuth in question are considered as received by the side lobes of the Sum channel and consequently eliminated, if:Gsum−GCont≦Threshold_RSLSCont 
A further consequence is that the responses received for the azimuth in question are considered as coming from transponders localized outside of the reception arc allowing the monopulse processing and consequently eliminated, if:GSum−GCont≦Threshold_RSLSDiff 
Consequently, the effective interrogation arc of the radar (1030 MHz), which corresponds to the region of space within which the transponder is capable of responding, is therefore limited:
for nearby aircraft, by implementing the ISLS function (described hereinabove);
for distant aircraft, by the penetration of the sensitivity threshold of the receiver of the transponder by the interrogation signals coming from the radar, which penetration is defined for the azimuth in question, by:GSum·Pint errogation−Losses≧Sensitivity_Threshold_Transponder,the Losses mentioned being mainly due to the losses resulting from the propagation of the signals between the radar and the aircraft.
It should be noted that the formula stated hereinabove is a simplified formula intended to highlight the role of the Gain of the antenna of the radar.
Similarly, the effective reception arc (1090 MHz) of the radar, which corresponds to the area within which the responses from the transponder will be processed by the radar, is on the other hand limited:
for nearby aircraft, by implementing the RSLS function (described hereinabove);
for distant aircraft, by the penetration of the sensitivity threshold of the receiver of the transponder by the interrogation signals coming from the radar, which penetration is defined for the azimuth in question, by:GSum·PResponses−Losses≧Sensitivity_Threshold_Transponder.
For distant aircraft, the above considerations in practice often lead, in the context of a long-range radar configuration, to the signals only being used both by the transponder and by the radar when the gain G of the antenna for the azimuth of the target with respect to the maximum gain of the antenna is such that:Gmax−G≦4 dB
Thus, in particular for operation in S mode or in IFF mode, the operational mode of the various channels of the secondary surveillance radar described previously, as regards the exchanges of information between the radar and the various transponders, leads to limitations in the use of the signals in transmission and in reception. The consequence of this is that, for a given transponder, the interrogation and response functions are limited in time to a time window defined by an angular opening corresponding to a fraction of the diagram of the Σ channel conventionally called illumination of the target. Outside of this window, the interrogations and the responses are not valid or are not taken into consideration.
Currently, these operational limitations are turning out to be increasingly incompatible with the operational limitations supported by the secondary surveillance radar systems responsible for the control of the air traffic, such as for military radar systems responsible for the IFF identification.
Indeed, in order to satisfy the need for closer approach of aircraft necessary due to the increasing density of flights, the only solution is to make the antennas of the radar rotate with higher and higher speeds in order to reduce the dead time between two orientations of the antenna lobe along the same azimuth and consequently between two successive detections of the same aircraft thus reducing the uncertainty in the position of an aircraft between two antenna rotations. Such an increase in the antenna rotation speed results in a further reduction in the interval of time during which a secondary surveillance radar can exchange information with a given aircraft, which interval of time is again referred to as illumination time of the target in question.
Furthermore, owing also to the increasing density of aircraft flights, the secondary surveillance civilian radar systems operating in S mode are obliged to selectively manage the routes of an increasing number of aircraft and must additionally, in mode EHS in particular, allow the exchange of an increasing quantity of data with the latter, which data are denoted “commB Data Selector registers” (or “BDS registers”) according to the terminology in current usage. The “BDS” are numbered according to the registers of the transponder, which registers contain the flight data. Thus, for example, the following can be differentiated: BDS40 (aircraft intention), the BDS50 (track and turn report) or again the BDS60 (heading and speed report).
For their part, military radar systems additionally require:
either a growing number of interrogations and of responses for each aircraft interrogated, in such a manner as to withstand jamming and intrusion systems that are becoming ever more sophisticated,
or a continually increasing illumination time on the target, in order to pick up responses with a long response time required by the new IFF modes of identification.
These growing limitations lead to constraints that are less and less compatible with the current operation of secondary surveillance radar systems.